The transmission control unit disclosed in the Bislew patent has met with a remarkable degree of commercial success and has been used for several years for most tractors manufactured by the assignee of the present invention. However, one problem has been detected in the hydraulic control system disclosed in the Bislew patent and relates to serviceability. In the control system disclosed in the Bislew patent, all of the valve components are incorporated into a single housing which means that if any one of the components, such as one of the valve bores, becomes defective, the entire unit must be replaced. This requires a substantial amount of time and is a fairly expensive proposition.
Another problem encountered in the control system disclosed in this patent is the fact that there is a small increment of delay when the transmission unit is shifted from one gear to another since the shifting requires de-pressurization of one friction device and pressurization of a second friction device which, of course, cannot be accomplished instantaneously.
Various other proposals have been made for controlling friction devices and these are disclosed in the following United States patents: U.S. Pat. Nos. 2,765,893, 2,792,714, 2,848,080, 2,991,862, 3,164,232, 3,237,734, 3,300,004, 3,459,071, 3,460,656, 3,656,600 3,674,121, 3,796,114, 3,823,621.